Business, science and entertainment applications depend upon computing systems to process and record data. In these applications, large volumes of data are often stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical, convenient, and secure means of storing or archiving data.
Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage capacities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is currently measured in hundreds of gigabytes.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. Later sensors using the GMR effect were developed. AMR and GMR sensors transduce magnetic field changes to resistance changes, which are processed to provide digital signals. AMR and GMR sensors offer signal levels higher than those available from conventional inductive read heads for a given read sensor width and so enable smaller reader widths and thus more tracks per inch, and thus higher data storage density. Moreover, the sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
When a tape is written to, the span of data just written is the span of the head elements. However, any expansion and contraction of the tape prior to reading results in an expansion or contraction of the space between data tracks and thus the data span. Present tapes typically expand and contract by approximately 1 part in 1000, or 0.1%.
In current Linear Tape Open (LTO) systems, the heads include servo readers that are approximately 3 mm apart. The tape media also includes servo tracks having a spacing of about 3 mm, thereby defining data bands of about 3 mm. A 0.1% expansion over 3 mm results in about 3 micrometers of expansion for a data band. Accordingly, the data tracks themselves must be greater than the reader widths plus 3 micrometers or the readback will suffer from expansion- or contraction-induced misregistration. Accordingly, current tape formats are reaching their limits as far as increasing track density is concerned. To illustrate, consider the following example.
In current tape head products, read sensor width is chosen to be ½ the track width on the tape. Assume that the tracks are 12 micrometers wide. The sensor is then 6 microns wide. If at the outer tracks, there are 3 micrometers of misregistration, then the readers over the outer data bands may be riding along the edge of the data band. Then the reader may come off the track due to uncompensated lateral tape excursions. Accordingly, the track widths (in this example) cannot be made smaller without increased risk of misreads due to tape wobble.
One method for compensating for tape lateral expansion and contraction is statically rotating the head and then making small angular adjustments to keep the readers/writers in the head aligned to tracks on the tape. However, the static rotation leads to skew-related misregistration and is generally complex and difficult to implement. For example tilted heads must be constructed so as not to steer tape, etc.
Another proposed solution attempts to control the tape width by controlling tape tension. However, this method works over a limited range only, and generally does not provide enough control.